Considering the original condition, a 10 ft. ladder resting on a wall with its top 8 ft. above the ground. A right angle triangle with hypotenuse of 10 ft. (length of ladder) and height of 8 ft. can be drawn, by applying pythagoras theorem we can derive the base of the triangle as 6 ft.After the ladder has shifted by 1 ft. away from the wall (as shown), we can conclude that the top of the ladder has also slides down the wall (lets assume that distance as "d"). In the new triangle, the hypotenuse remains 10 ft., the base becomes (6+1) = 7 ft. and the height will be now (8-d) ft. Again on applying pythagoras theorem, we can calculate d=~0.86 ft., which is less than 1.